Thanks Alex again.
I had tried your method but it didn't change anything.
But I found that: Some apps are working but not all.
I'm now trying receiver examples from:
http://www.oz9aec.net/index.php/gnu-radio/grc-examples
For example, ssb_rx.py doesn't work but ssb_rx_play works fine.
So I guess maybe it's because the source code I down from oz9aec.net is
something wrong. I'm trying to figure out where the problem is.
Thanks for all and best wishes.
Qin Jian